System and method for protecting against magnetic fields produced by MRI

ABSTRACT

An implantable cardiac device that detects and protects against strong magnetic fields produced by MRI equipment is disclosed. The device has a magnetic field sensor for detecting the presence of a relatively weak static magnetic field ( 102, 110, 118, 122 ) of a level equivalent to that of a permanent magnet in the vicinity of the device. The device is switched from a standard operating mode ( 100 ) where the nominal functions of the device are active, to a specific protected MRI mode ( 114, 116 ) in the presence of a magnetic static field of a level corresponding to that emitted by MRI equipment. The device further temporarily switches the device from the standard operating mode ( 100 ) to an MRI stand-by state ( 108 ) when a magnetic field is detected by the magnetic field sensor such that a subsequent detection of a magnetic field switches the device from an MRI stand-by state to the specific protected MRI mode.

The present application claims the benefit of French Application No.10-50374 entitled “Implantable cardiac prosthesis comprising means ofdetection and of protection against strong magnetic fields produced byMRI” and filed Jan. 20, 2010, which is hereby incorporated by referencein its entirety.

FIELD

The present invention relates to “active implantable medical devices” asdefined by Directive 90/385/EEC of 20 Jun. 1990 of Council of EuropeanCommunities, and more particularly to devices that continuously monitorheart rhythm and if necessary deliver to the heart of a patientresynchronization and/or defibrillation electrical stimulation pulses,in cases of arrhythmias detected by the device.

The present invention relates even more particularly to techniques forprotecting these devices (e.g., generators and their associated sensors)when the patient is subject to an examination by magnetic resonanceimaging (MRI) equipment.

BACKGROUND

Active implantable medical devices include a housing containing variouselectronic circuits and a battery, generally referred to as a generator,that is electrically and mechanically connected to one or more leadswith electrodes. The leads are intended to come into contact with atissue of the patient, e.g., the myocardium, at sites where electricalpotentials can be collected (i.e., detected) and/or stimulation pulsescan be applied (i.e., delivered).

MRI examination has been contraindicated for patients with an implantedcardiac pacemaker or defibrillator type of generator. Several types ofproblems arise under this situation:

heating near the electrodes connecting the generator to the patient'sheart;

forces and torques of attraction exerted on the device immersed in thehigh intensity static magnetic field produced by the MRI equipmentduring an examination; and

unpredictable behavior of the device itself due to the exposure to thestrong magnetic field.

It is an objective of the present invention to provide a solution to theabove-described problems, particularly to avoid malfunction orunpredictable behavior of the device in a strong magnetic field. In thisregard, it is desirable that when the device is exposed to (static andalternating) electromagnetic fields generated by MRI equipment, thebehavior of the device is documented and known in advance.

In the absence of special precautions, the problems that are likely toaffect the behavior of the device under an MRI examination include anerratic detection of strong static field generated by MRI equipment,especially in devices equipped with a Reed magnetic switch. A Reedmagnetic switch is used to detect and respond to the presence of apermanent magnet in the vicinity of the device. A permanent magnet isnormally used by a practitioner to put the device in a safe operating or“magnet” mode, for example, when using an electric scalpel, orevaluating battery depletion of the device. In a magnet mode, thestimulation frequency is generally fixed and reflects the level ofbattery charge. A Reed magnetic switch is designed to detect staticmagnetic fields of relatively low intensity, but is likely to exhibit atotally unpredictable behavior in an MRI examination environment wherethe magnetic fields are often thousands of times stronger than that of apermanent magnet. Problems also may include deterioration of theintrinsic performance of the device, and misinterpretation of thedynamic signals emitted by the MRI equipment by the device as cardiacsignals, including, for example, an inhibition of the stimulationfunction by the dynamic signals emitted by the MRI equipment that areinadvertently detected by the device as cardiac signals.

Throughout the duration of an MRI examination—which can last severalminutes—the device should nevertheless remain functional and provide ifnecessary seamless and predictable stimulation to the patient'smyocardium. It is therefore desirable to have means for detecting andmeans for managing such a situation, providing the following functions:

indicating to the device that the patient will be subjected to an MRIexamination;

inhibiting the circuits of the device that may be disturbed by theelectromagnetic fields emitted by the MRI equipment; and

operating the device in a dedicated pacing mode, tailored to the patientand compatible with the electromagnetic fields emitted by the MRIequipment.

Special techniques have been proposed to detect static magnetic fieldsof an MRI type, with strength in the order of Tesla (typically between0.5 and 3 T, and herein referred to as a “strong magnetic field”): Inthe majority of the devices that are sensitive to the presence of apermanent magnet are those devices that have a magnetic field detectorthat detects weak magnetic fields (e.g., a static magnetic field in theorder of 1.5 mT; and herein referred to as a “weak magnetic field”), butis unable to detect the strong magnetic fields that are produced by MRIequipment. Because the magnetic fields produced by MRI equipment are upto thousands of times stronger than that produced by a permanent magnet,and are in the non linear response zone of these weak magnetic fielddetectors, it poses a risk that the weak magnetic field detectors willbe “deaf” to the presence of a strong magnetic field.

The U.S. Patent Publication US 2007/191914 A1 describes a device inwhich the presence of a strong static magnetic field is detected by ananalysis of the impedance of an inductive component, e.g., one of thecoils of an inductive switching regulator. The presence of a strongmagnetic field has the effect of saturating the core of this inductivecomponent, causing an impedance change that can be reported to thedevice.

WO 2006/124481 A2 discloses another technique for detecting the presenceof an MRI-type magnetic field by the measurement of the voltage sensedacross a telemetry antenna and on the lead.

EP 1935450 A1 describes yet another technique of using giantmagnetoresistance (GMR) sensors associated within a Wheatstone bridge.The Wheatstone bridge acts as a single mixed strong/weak field sensor.The balance of the Wheatstone bridge is more or less altered by amagnetic field, and the resulting changes from the differential voltagecan be analyzed by a converter placed at the output of the bridge togive an overall estimate of the field strength.

The U.S. Patent Publication US2009/0138058 A1 provides a programmingtechnique to place the device in a state of waiting for MRI (referred toas an MRI mode), for example, during a consultation with a cardiologist.In this state of waiting, the device operates in its standard mode ofoperation, but with an expectation of detecting a strong static magneticfield. In the presence of a strong magnetic field (e.g., at thebeginning of an MRI examination), the device then switches to anMRI-safe mode that is compatible with a strong magnetic field for theduration of the MRI examination. Once the MRI examination is completed,the device is reset to a normal mode of operation (referred to as anormal operation mode).

Some of these various techniques detect a strong magnetic field using anadditional sensor (i.e., not the sensor responsive to a weak magneticfield). If these devices do not contain a strong magnetic field sensor,it is required to redesign the hardware of the device, incurring anadditional cost and creating an extra constraint on the circuits againstthe design requirements for miniaturizing these devices.

OBJECT AND SUMMARY

It is, therefore, an object of the present invention is to provide a newtechnique particularly applicable to conventional and/or oldergeneration devices, with no dedicated sensor for detecting a strongmagnetic field generated by MRI equipment, to protect such devicesduring an examination by the MRI equipment.

It will be seen in particular that the technique of the presentinvention can be implemented by a simple software adaptation to anexisting microcontroller of an active implantable medical device withouthaving to modify the hardware design, and thus provides a large economyof savings in connection with implementing the invention with nosignificant extra cost.

The basic principle of the present invention is to use a magnetic fieldsensor that is present in the device for detecting a weak magnetic field(typically a low static magnetic field in the order of millitesla, mT),such as is generated by a permanent magnet used to place the device in a“magnet mode”. The magnetic field sensor is used without hardwarechanges to the device for placing the device in a state of waiting forMRI (also referred to as “stand-by state”), for example, during acardiac consultation.

When a patient undergoes an MRI examination, an appointment is given bya radiology center. Prior to the appointment, the patient meets with hisor her cardiologist, who places the device in a stand-by state for anMRI examination.

In a stand-by state for MRI, the operation of the device is initially ina standard operation mode, until the occurrence of a trigger events suchas one of the following two events:

(1) when the patient enters the examination room where MRI equipment islocated, the device detects a relatively high static magnetic field:although significantly weaker than the strong magnetic field that willbe generated by the MRI equipment during the actual examination, such arelatively higher static field is present in the examination room evenwhen no test is in progress; and(2) just before the entrance of the patient in the room of the MRIequipment, a permanent magnet is applied by an operator to the patient'schest in the vicinity of the device.

Either of these two events thus places the device in a specific mode ofoperation, modified from the standard operation mode that is compatiblewith a strong magnetic field during the MRI examination. It is notedthat the transition to the specific mode of operation is made justbefore the beginning of the MRI examination, either automatically uponentering the examination room or by a special programming (e.g.,application of a permanent magnet).

Once the MRI examination is completed and the level of ambient magneticfield has dropped to a lower level, e.g., when the patient has left theexamination room, the device automatically detects the change in themagnetic field level and returns to its normal operation mode, withoutany necessary intervention such as by reprogramming the device asdescribed in US2009/0138058 A1.

In one embodiment, when the device is switched to the MRI stand-bystate, the cardiologist may program a timer with a predetermined period(e.g., a few hours, several weeks). When the timer is activated by suchprogramming, the device is put in the MRI stand-by state. After thepredetermined period, for safety purpose, the device automatically exitsthe MRI stand-by state without the need of any additional consultationof the cardiologist.

One embodiment of the present invention is directed to a device of thetype disclosed in US2009/0138058 A1 that comprises: a generatorcontaining an electronic circuit of detection/stimulation; means forswitching the device from a standard operating mode where nominalfunctions of the device are active to a specific protected MRI mode inthe presence of a strong magnetic field of a level equivalent to thestatic magnetic field emitted by MRI equipment; emitting/receivingtelemetry for ensuring a coupling of the device with an externalprogrammer device; and means for temporarily setting the device in anMRI stand-by state.

Characteristically of the present invention, the device includes amagnetic field sensor detecting the presence of a weak static magneticfield equivalent to that emitted by a permanent magnet in the vicinityof the device. Moreover, the device is returned from the specificprotected MRI mode back to the standard operating mode, in the absenceof that weak magnetic field being sensed by the magnetic field sensor.

Advantageously, the means for switching to place the device in the MRIstand-by state is activated upon detecting a magnetic field (e.g.,entrance to the MRI examination room, application of a permanent magnetin the vicinity of the device) by the magnetic field sensor. The meansfor switching may be alternatively activated via transmitter/receivertelemetry means.

Further, means for repositioning may be provided to return the devicefrom operating in the MRI stand-by state or the specific protected MRImode back to the operating standard mode. Such a means for repositioningmay be activated via the transmitter/receiver telemetry means.

According to one embodiment, the device further comprises a timer forcounting a predetermined time from the moment the device is placed inthe MRI stand-by state, and means for automatically placing the devicein the standard operating mode at the expiry of the timer. Thepredetermined time may be programmable data by an external programmerand transmitted to the device via the transmitter/receiver telemetrymeans.

Preferably, the device further comprises means for inhibiting the switchback of the device from the specific protected MRI mode to the standardoperating mode, the means for inhibiting operating during apredetermined period counted from the moment of the switching in thespecific protected MRI mode. This timing may be a predetermined timeperiod programmed by the external programmer device and transmitted tothe device via the transmitter/receiver telemetry means.

The magnetic field sensor is typically a weak magnetic field sensoradequate for detecting the presence of a static magnetic field of alevel of at least 1 mT. In one embodiment, it is selected from the groupconsisting of: a coil whose core tends to saturate in the presence aweak magnetic field, a giant magnetoresistance GMR sensor, a Hall effectsensor, an integrated MAGFET sensor, and a MEMS sensor.

The means for switching includes, when activated, means for putting thedevice into a specific mode that is protected against MRI, such as amagnet mode without detection; means for inhibiting the elements of thedevice sensitive to the deleterious effects when exposed to static andalternating radiofrequency magnetic fields emitted by MRI equipment; andmeans for generating pacing pulses at a predetermined frequency,independent of the battery level of the power supply of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, characteristics and advantages of the presentinvention will become apparent to a person of ordinary skill in the artfrom the following detailed description of various embodiments of thepresent subject matter made with reference to the annexed drawings, inwhich:

FIG. 1 is a schematic view of an implantable medical device, accordingto a preferred embodiment of the invention; and

FIG. 2 is a diagram of the operation of the device of FIG. 1, accordingto one embodiment of the present invention.

DETAILED DESCRIPTION

The present invention may be implemented by an appropriate programmingof the controlling software of a known device of, for example, a cardiacpacemaker, resynchronizer and/or defibrillator type, including means foracquiring a signal provided by endocardial leads and/or one or moreimplanted sensors. The adaptation of these devices to implement thefunctions of the present invention is believed to be within theabilities of a person of ordinary skill in the art, and therefore willnot be described in detail.

The present invention may particularly be applied to implantable devicessuch as those of the Reply and Paradym families produced and marketed bySorin CRM, Clamart, France. These devices include programmablemicrocontroller and microprocessor circuitry to receive, format, andprocess electrical signals collected by implanted electrodes and deliverpacing pulses to these electrodes. It is possible to transmit bytelemetry software and to store the software in the memory and executethe software to implement the functions of the present invention asdescribed herein.

With reference to FIG. 1, the housing 12 of an implantable medicaldevice 10 contains a battery 14 and an electronic circuit 16. Theelectronic circuit 16 is connected to a connector 18 that removablyreceives one or more leads (not shown) having electrodes for detectionand/or stimulation.

The device 10 is also equipped with a magnetic field sensor 20 fordetecting a weak static magnetic field, typically in the order ofmillitesla (mT).

The commonly used techniques for detecting a permanent magnet place thedevice 10 in a specific “magnet mode”. The sensor 20 may be chosen fromamong the group comprising: a coil whose core saturates in the presenceof a weak magnetic field, a giant magnetoresistance (GMR) sensor, a Halleffect sensor, an integrated MAGFET component, (as described by examplein WO 94/12238 A1), or a micro electromechanical system (MEMS) sensor(as described, for example, in FR 2805999 A1). A Reed magnet switch thatcontains metal elements is preferably avoided because of its generallyunpredictable and unreliable behavior in the presence of strong magneticfields.

The sensor 20 contained in the device 10 advantageously allows forimplementing the present invention without any hardware changes to thedevice 10, and for retrofitting already implanted devices. The sensor 20outputs a binary ‘true’ or ‘false’ CCM signal, depending on whether amagnetic field stronger than a threshold level is detected. Thethreshold level is typically greater than or equal to 1 mT.

FIG. 2 is a flowchart explaining a detailed implementation of thetechnique according to the present invention.

In the initial state (block 100), the device 10 is in a standard(unmodified) operation mode. A binary indicator EAIRM (corresponding tothe MRI stand-by state) is set to ‘false’.

According to one embodiment, upon detecting a magnetic field (CCM=‘true’in test 102) by the sensor 20, the device 10 goes into a “magnet mode”(block 104). In the magnet mode (also referred to as a ‘standard magnetmode’), the patient is paced in a conventional DOO mode with astimulation frequency that is determined based on the level of batterydepletion (which allows testing of that level). Other conventionalasynchronous pacing modes can be used as deemed appropriate for thedevice used and the patient, e.g., VOO, AOO.

When the magnetic field is removed (CCM=‘false’ in test 102), the device10 returns to the programmed standard mode of operation. The operationin a standard mode, which may actually comprise a number of differentstandard operating modes responsive to the patient's condition, is wellknown in the art.

According to another embodiment, in the standard mode (block 100), thedevice 10 receives a command 106 via telemetry (telemetry or inductivetelemetry RF), e.g., on the orders of a cardiologist. The command 106switches the device 10 to the MRI stand-by state (block 108).

The MRI stand-by state is maintained while the patient undergoes an MRIexamination during a period of time, typically between a few minutes anda few hours. The MRI stand-by state may be extended to a few days toweeks as the cardiologist determines appropriate. The EAIRM indicator isset to ‘true’, and the MRI stand-by state time is programmed by thecardiologist, up to the maximum allowable time that the device 10 canremain in the MRI stand-by state. A countdown timer starts immediatelyafter the device 10 switches to the MRI stand-by state. Alternatively,the countdown timer may be deferred for another programmable periodbefore the device 10 switches to the MRI stand-by state.

In the MRI stand-by state, the device 10 operates with all the usualfeatures, for example, in the same standard operating mode of the mainblock 100.

The standard operating mode is maintained until the magnetic fieldsensor 20 detects a static field (test 110, CCM=‘false’). If, however,(i) the sensor 20 detects the presence of a magnetic field (test 110,CCM=‘true’) and (ii) the countdown timer has not expired (test 112), thedevice 10 considers that the patient will be subjected soon to a strongmagnetic field.

The device 10 changes its electronic behavior (block 114) by turning offa number of circuits directly or indirectly, especially the RF telemetrycircuits and the switching converters for power supply. The powersystems are based on linear voltage regulators or capacitive converters,consuming more energy, but insensitive to the effects of magneticfields.

After changing its electronic behavior, the device 10 changes itsoperating mode (block 116) to a ‘modified magnet mode’ in which:

the pacing frequency is no longer based on the battery depletion (as inthe ‘standard magnet mode’), for example, on the average of thepatient's heart rhythm;

the detection function is inhibited to avoid misinterpretation of thedynamic signals emitted by the MRI equipment as cardiac signals; and

in the case of an implantable defibrillator, any delivery of shocks isinhibited.

The ‘modified magnet mode’ (also referred to herein as a specificprotected MRI mode) is maintained during the MRI examination.

The test for detecting the magnetic field is repeated (test 118). If thetest is positive (i.e., CCM=‘true’), the device 10 maintains themodified magnet mode. Otherwise the test is negative (i.e.,CCM=‘false’), and the device 10 waits for the expiration of aprogrammable confirmation time (DCP), for example, 10 minutes (block120), after which the test for the presence of the magnetic field isrepeated (test 122). If this test is positive (i.e., CCM=‘true’), thedevice 10 remains in the current modified magnet mode; otherwise (i.e.,CCM=‘false’), the device 10 switches to the standard electronicoperation (block 124) restoring the altered functions in block 114, andthe modified magnet mode is abandoned (block 126) restoring the alteredfunctions in block 116. The functions and the mode of operations thatprevailed before detecting a magnetic field during test 110 are returnedto the MRI stand-by state with standard operation of block 108.

When the device 10 is in the MRI stand-by state (block (108)), if (i)the sensor detects the presence of a magnetic field (test 110,CCM=‘true’) but (ii) the timer is expired (test 112), the MRI stand-bystate is cancelled (block 128, EAIRM indicator set to ‘false’). Thedevice 10 reverts to the ‘standard magnet mode’ (block 104), and thetest 102 for testing the presence of the field is repeated. If this test102 is negative (CCM=‘false’), the device 10 returns to the standardmode corresponding to the initial state of operation (block 100) priorto the activation of the MRI stand-by mode by the cardiologist.

In another embodiment, the device 10 is forced to return to the standardstate by a telemetry instruction 130 before the timer expires. In thiscase the timer is disabled and is not taken into account.

Note that when the device 10 is in the MRI stand-by state (block 108),the device 10 switches to the protected modified magnet mode either bythe automatic detection of a relatively high ambient magnetic fieldprevailing in the MRI examination room as described above, or byapplication of a permanent magnet by an operator on the patient's chestin the region of the device (132).

The above described procedures are particularly recommended with a useof MRI equipment to scan a part of the patient's body (e.g., limbs,head, legs, arms), because during the scanning the patient wears ashielding cloth protecting his trunk from strong dynamic fields, whichmight prevent an automatic detection of the strong static field emittedby the MRI equipment. The practitioner has an opportunity to force theswitching of the device 10 in the modified electronic operation and themodified magnet mode by forcing the test 110, using a permanent magnetbefore the patient dons the protective shielding cloth.

One skilled in the art will appreciate that the present invention may bepracticed by other than the embodiments disclosed herein, which areprovided for purposes of illustration and not of limitation.

The invention claimed is:
 1. An active implantable medical device for cardiac pacing, resynchronization and/or defibrillation, comprising: an electronic circuit configured to provide electrical stimulation signals at a stimulation frequency; and a magnetic field sensor configured to detect the presence of a predetermined permanent magnet, wherein the magnetic field sensor is configured to detect the presence of the permanent magnet by comparing a level of a first magnetic field detected by the magnetic field sensor to a single preset threshold magnetic field level set to a level equivalent to that emitted by the predetermined permanent magnet, wherein the first magnetic field is a static magnetic field that is weaker than a second magnetic field emitted by the MRI equipment while being operated during an MRI examination; wherein the electronic circuit is configured to switch the active implantable medical device between a standard operating mode and a mode awaiting the MRI examination; wherein the electronic circuit and magnetic field sensor are configured to provide switching between at least four different modes of operation of the active implantable medical device using the detection of the permanent magnet, wherein the electronic circuit is configured to determine a mode of operation of the at least four different modes of operation to which to switch based on a current mode of operation of the active implantable medical device when the magnetic field sensor detects the presence of the permanent magnet using the single preset threshold magnetic field level; wherein, when the active implantable medical device is in the mode awaiting the MRI examination, the electronic circuit is configured to switch the active implantable medical device from the mode awaiting the MRI examination to a protected operating mode in response to the magnetic field sensor detecting the presence of the predetermined permanent magnet, wherein the protected operating mode is configured to protect the active implantable medical device from effects of the stronger second magnetic field generated by the MRI equipment during the MRI examination; and wherein, when the active implantable medical device is in the standard operating mode, the electronic circuit is configured to switch the active implantable medical device from the standard operating mode to a magnet mode in response to the magnetic field sensor detecting the presence of the predetermined permanent magnet, wherein, in the magnet mode, the electronic circuit adjusts the stimulation frequency based on a level of consumption of a power supply battery.
 2. The device of claim 1, further comprising: transmitter/receiver telemetry providing communication coupling of the device with an external programmer device, wherein the electronic circuit is configured to receive a command to temporarily place the device in the mode awaiting the MRI examination via the transmitter/receiver telemetry.
 3. The device of claim 1, further comprising: transmitter/receiver telemetry providing communication coupling of the device with an external programmer device, wherein the electronic circuit is configured to receive a command to switch the device from the mode awaiting the MRI examination to the standard operating mode via the transmitter/receiver telemetry.
 4. The device of claim 1, wherein the magnetic field sensor detects the presence of a static magnetic field of at least 1 mT.
 5. The device of claim 1, wherein the magnetic field sensor is selected from a group consisting of: a coil whose core saturates in the presence of a weak magnetic field, a giant magnetoresistance (GMR) sensor, a Hall effect sensor, an integrated MAGFET sensor, and a MEMS sensor.
 6. The device of claim 1, wherein, in the protected operating mode, the electronic circuit is configured to: inhibit detection of cardiac signals; inhibit elements of the device sensitive to deleterious effects of exposure to static and alternating radio frequency magnetic fields emitted by MRI equipment; and generate pacing pulses at a predetermined frequency, independent of the level of a battery of the device.
 7. The device of claim 1, further comprising: a timer for counting a predetermined time period from a moment when the device is placed in the mode awaiting the MRI examination, wherein the electronic circuit is configured to automatically place the device in the standard operating mode at the expiry of the timer.
 8. The device of claim 7, further comprising: transmitter/receiver telemetry providing communication coupling of the device with an external programmer device, wherein the predetermined time period of the timer is programmable by said external programmer device and transmitted to the device via the transmitter/receiver telemetry.
 9. The device of claim 1, wherein the electronic circuit is configured to inhibit switching of the device from the protected operating mode to the standard operating mode during a predetermined period after the switching of the device into the protected operating mode.
 10. The device of claim 9, further comprising: transmitter/receiver telemetry providing communication coupling of the device with an external programmer device, wherein said predetermined period is programmable by said external programmer device and transmitted to the device via the transmitter/receiver telemetry.
 11. An active implantable medical device configured to perform at least one of cardiac pacing, resynchronization, or defibrillation, the active implantable medical device comprising: a power supply battery configured to provide power to the active implantable medical device; an electronic circuit configured to provide electrical stimulation signals at a stimulation frequency; and a magnetic field sensor configured to measure a strength of a first magnetic field detected by the magnetic field sensor; wherein at least one of the electronic circuit or the magnetic field sensor are configured to detect a presence of a predetermined permanent magnet by comparing the strength of the first magnetic field detected by the magnetic field sensor to a single preset threshold magnetic field level set to a level emitted by the predetermined permanent magnet, wherein the first magnetic field that the magnetic field sensor is configured to detect is weaker than a second magnetic field generated by MRI equipment during an MRI examination; wherein the electronic circuit is configured to switch the active implantable medical device between a standard operating mode and a mode awaiting the MRI examination; wherein the electronic circuit and magnetic field sensor are configured to provide switching between at least four different modes of operation of the active implantable medical device using the detection of the predetermined permanent magnet, wherein the electronic circuit is configured to determine a mode of operation of the at least four different modes of operation to which to switch based on a current mode of operation of the active implantable medical device when the magnetic field sensor detects the presence of the predetermined permanent magnet using the single preset threshold magnetic field level; wherein, when the active implantable medical device is in the mode awaiting the MRI examination, the electronic circuit is configured to switch the active implantable medical device from the mode awaiting the MRI examination to a protected operating mode in response to the at least one of the electronic circuit or the magnetic field sensor detecting the presence of the predetermined permanent magnet, wherein the protected operating mode is configured to protect the active implantable medical device from effects of the stronger second magnetic field generated by the MRI equipment during the MRI examination; and wherein, when the active implantable medical device is in the standard operating mode, the electronic circuit is configured to switch the active implantable medical device from the standard operating mode to a magnet mode in response to the at least one of the electronic circuit or the magnetic field sensor detecting the presence of the predetermined permanent magnet, wherein, in the magnet mode, the electronic circuit adjusts the stimulation frequency based on a level of consumption of the power supply battery.
 12. The device of claim 11, wherein the permanent magnet is a separate device from non-operating MRI equipment.
 13. The device of claim 11, wherein the first magnetic field detected by the magnetic field sensor comprises a static magnetic field, and wherein the second magnetic field generated by the Mill equipment during the MRI examination comprises both a static magnetic field and an alternating magnetic field.
 14. The device of claim 11, wherein the magnetic field sensor is configured to detect an absence of the first magnetic field after detecting presence of the first magnetic field, and wherein the electronic circuit is configured to switch the active implantable medical device from the protected operating mode to the standard operating mode in response to the magnetic field sensor detecting the absence of the first magnetic field.
 15. The device of claim 11, wherein the magnetic field sensor is configured to detect the presence of the first magnetic field and not the second magnetic field generated by the MRI equipment during the MRI examination.
 16. The device of claim 11, wherein the predetermined threshold magnetic field level is 1 mT.
 17. The device of claim 16, wherein a strength of the first magnetic field is less than 1 mT.
 18. The device of claim 16, wherein a strength of the second magnetic field is at least 0.5 T.
 19. A method comprising: measuring a strength of a first magnetic field detected by a magnetic field sensor of an active implantable medical device, wherein the active implantable medical device is configured to be switched between a standard operating mode and a mode awaiting an MRI examination; detecting a presence of a predetermined permanent magnet by comparing a level of the first magnetic field detected by the magnetic field sensor to a single predetermined threshold magnetic field level set to a level emitted by the predetermined permanent magnet, wherein the first magnetic field that the magnetic field sensor is configured to detect is weaker than a second magnetic field generated by MRI equipment during the MRI examination; switching between at least four different modes of operation of the active implantable medical device using the detection of the permanent magnet, wherein the electronic circuit is configured to determine a mode of operation of the at least four different modes of operation to which to switch based on a current mode of operation of the active implantable medical device when the magnetic field sensor detects the presence of the permanent magnet using the single preset threshold magnetic field level; switching the active implantable medical device from the mode awaiting the MRI examination to a protected operating mode in response to detecting the presence of the predetermined permanent magnet when the active implantable medical device is in the mode awaiting MRI examination, wherein the protected operating mode is configured to protect the active implantable medical device from effects of the stronger second magnetic field generated by the MRI equipment during the MRI examination; protecting the active implantable medical device from effects of the stronger second magnetic field generating by the MRI equipment during the MRI examination when the active implantable medical device is in the protected operating mode; switching the active implantable medical device from the standard operating mode to a magnet mode in response to detecting the presence of the predetermined permanent magnet when the active implantable medical device is in the standard operating mode; and adjusting the stimulation frequency based on a level of consumption of the power supply battery when the active implantable medical device is in the magnet mode.
 20. The method of claim 19, wherein the permanent magnet is a separate device from non-operating MRI equipment.
 21. The method of claim 19, wherein the predetermined threshold magnetic field level associated with the permanent magnet detected by the magnetic field sensor is 1 mT, and wherein the strength of the second magnetic field generated by the MRI equipment during the MRI examination is at least 0.5 T. 